1. Field of the Invention
The present invention relates to a method and apparatus for controlling air-fuel ratio learning of an internal combustion engine, and more particularly to control technology for modifying a method of updating/setting treatment of an air-fuel ratio learning correction value in accordance with acquisition conditions of air-fuel ratio feedback correction values.
2. Description of the Related Art
As conventional air-fuel ratio feedback controllers incorporating a learning function, there are for example the devices disclosed in Japanese Unexamined Patent Publication Numbers 60-90944, and 61-190142.
With these devices, air-fuel ratio feedback control involves determining the richness/leanness of the actual air-fuel ratio with respect to a target air-fuel ratio (for example the stoichiometric air-fuel ratio), by comparing an output value of an oxygen sensor provided in an engine exhaust system with a slice level (a value corresponding to a target air-fuel ratio), and then incrementing/decrementing an air-fuel ratio feedback correction coefficient .alpha. by proportional/integral control and the like, based on the determined results. A basic fuel injection quantity Tp computed from, the intake air flow quantity detected by an air flow meter, and the engine rotational speed, is then corrected with the air-fuel ratio feedback correction coefficient .alpha., to minimize a deviation of the actual air-fuel ratio from the target air-fuel ratio, due for example to component errors and deterioration with time, or to environmental changes.
Moreover, the learning function involves, updating/storing the deviation of the air-fuel ratio feedback correction coefficient .alpha. from a reference value (target convergence value), as an air-fuel ratio learning correction coefficient K.sub.L (air-fuel ratio learning correction value) for each of several partitioned engine operating regions (that is, learning areas). The basic fuel injection quantity Tp is then corrected with the air-fuel ratio learning correction coefficient K.sub.L, so that a base air-fuel ratio obtained without the air-fuel ratio feedback correction coefficient .alpha., coincides approximately with the target value, thus enabling a more rapid convergence in the air-fuel ratio feedback control, of the actual air-fuel ratio on the target air-fuel ratio.
That is to say, by incorporating a learning function in the air-fuel ratio feedback control, then the actual air-fuel ratio can be better controlled to close to the target air-fuel ratio, corresponding with good response to correction requirements for the fuel injection quantity which differ for each operating condition.
With the abovementioned conventional air-fuel ratio learning control apparatus however, in order to improve the learning accuracy when updating/setting the learning correction coefficient K.sub.L (in other words, so that learning can be carried out under conditions wherein the air-fuel ratio feedback control is stable), if in a predetermined learning area, the oxygen sensor output exceeds the slice level for a predetermined number of times (for example two times) or more, then the deviation of the air-fuel ratio feedback correction coefficient .alpha. from the reference value during the subsequent period wherein the oxygen sensor output exceeds the slice level for a predetermined number of times (at least twice), is used in the computation for the learning correction coefficient K.sub.L. As a result, the following problems can arise.
Since the comparison of the oxygen sensor output value with the slice level is made for example for each input of a reference signal generated corresponding to each cylinder piston reference position, then in a region such as a low rotational speed idling region and the like, the number of times the oxygen sensor output exceeds the slice level within a predetermined period will be less than for a high rotational speed region. As a result, the learning opportunity particularly in the idling region is reduced, so that learning is not expedited, and learning accuracy is thus reduced.
Moreover, in the idling region the exhaust flow rate is inherently low. As a result, due to the poor response characteristics of the oxygen sensor in regions of low exhaust flow rates, the rich/lean inversion period is increased, further promoting the beforementioned reduction in learning opportunity.